Automotive electric power steering devices (EPS) employ a speed reduction apparatus. A column-type electric power steering device, for example, is adapted to reduce the rotation speed of an output shaft of an electric motor through a worm shaft and a worm wheel to amplify the output of the electric motor and transmit the output to a steering mechanism, thereby providing torque assist for a steering operation.
In general, a cylindrical joint which connects the output shaft of the electric motor to the worm shaft provides spline-coupling to an end of the worm shaft. The spline-coupled portion has a slight play defined in a rotation direction, so that noise occurs due to rattling attributable to the play.
To cope with this, an electric power steering device is provided in which an output shaft of an electric motor is coupled to a worm shaft via a joint including an elastic member for power transmission (see, for example, Japanese Unexamined Patent Publication No. 2002-145083 disclosed by Japanese Patent Office on May 22, 2002).
The elastic member is disposed between a pair of co-rotatable iron engagement members at opposed ends of the output shaft and the worm shaft with an interference. If the interference is great, it is difficult to fix the elastic member between the engagement members, and a loss torque due to frictional resistance is increased to deteriorate a steering feeling.
On the other hand, if the interference is small, the elastic member is worn during prolonged use, so that gaps are formed between the elastic member and the engagement members. Therefore, the joint has plays defined in a rotation direction due to the gaps. As a result, noise and torque transmission variation occur to deteriorate the steering feeling.
Further, the respective components have different dimensional tolerances, making it very difficult to properly determine the interference.
It is an object of the present invention to provide an electric power steering device which is easier to assemble, has a relatively small loss torque, and suppresses rattling and noise for a long period of time.